1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning device can comprise an array of individually controllable elements that serve to generate the circuit pattern. This is known as a maskless lithographic apparatus.
An array of lenses can be arranged perpendicularly to the direction of propagation of the projection beam, each lens transmitting and focusing a unique part of the projection beam onto the substrate. This is known as a micro lens array (MLA).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
When exposing substrates it is important that the patterned projection beam is accurately focused onto the substrate. Changes in the thickness or warping of the substrate can lead to inaccuracies in the exposure of substrate. This has conventionally been remedied using actuators to adjust the height of the substrate. The disadvantage of this solution is that it is necessary to move or bend the entire substrate.
Maskless apparatus are often used in the manufacture of flat panel displays. Substrates used for flat panel displays are more flexible than conventional silicon substrates, and therefore more susceptible to deviation in the height of the surface on which the image is to be exposed. When exposing flat panel displays, it is also desirable to be able to change the image being exposed.
Therefore, what is needed is a system and method that provide an improved method of compensating for changes in thickness of a substrate.